1. Field of the Invention
The present invention relates to a system for measuring a current position of an automobile or the like by using a global positioning system (GPS) and a receiver for the GPS system.
2. Description of the Related Art
A great deal has been placed on an NAVSTAR (Navigation System with Time and Range)/GPS which has been developed and used in the U.S. Army, because this system is capable of measuring a position of a target object at any place and time in the world. In recent days, a GPS receiver unit as well as a car navigation system provided with the GPS receiver have been made commercially available. In the near future, the GPS receiver may be used in other commercial applications.
The GPS known by the present inventors is designed to capture an electromagnetic wave transmitted from a GPS satellite going around the earth and maintaining a highly accurate orbit, measure the time when the wave reaches the GPS, and measure the distance between the GPS satellite and the receiving mobile station (user) based on the measured time. The similar measurement is carried out in the other two GPS satellites for deriving the position of the mobile station.
The unknown position of the mobile station can be derived by the positions of the three GPS satellites obtained from the orbit information and the measured distances from the station to each GPS satellite.
Even if the foregoing process is carried out with respect to these three GPS satellites, the measured time has an uncertain component. Hence, by treating a time lag .DELTA. t as one variant and capturing the positions of the four GPS satellites, it is possible to accurately measure the position of the GPS mobile station.
The GPS has a capability of measuring the position with high precision of about several tens to hundred of meters. Hence, the mobile station is capable of grasping the position of itself and the base station for managing two or more mobile stations loaded on a car, and for example, is also capable of managing the position of each mobile station. The managing result is used for managing an everyday operation range or providing a service for a user.
The following description will be directed to the known GPS position measuring system.
A GPS signal is received through a receiving antenna. The received signal is sent to an amplifying and frequency converting circuit in which the signal is amplified and then is converted into a signal on an IF (Intermediate Frequency) band (referred to as an IF signal). The resulting signal is sent to an analog-to-digital converter. In the A/D converter, the signal is directly converted into a digital signal or the base-band signal is converted into a digital signal. The digital signal is sent to a digital signal processing circuit in which a spectrum back-diffusion process, a digital demodulating process and a position measuring process are carried out with respect to the digital signal for deriving the data about the measured distance and the satellite orbit.
Four GPS satellites are required for measuring the position of a target object. Hence, at first, four satellites are selected from all GPS satellites which can be captured. As a determining criterion for the selection, a GDOP (Geometrical Dilution of Precision) is used. The GDOP is a coefficient standing for a geometrical precision degradation.
In the selecting process, at first one GPS satellite is captured so that a signal received from the captured GPS satellite is converted to the corresponding digital signal and collection of pieces of data about orbits of all the GPS satellites is performed. The collective data are referred to as an "almanac". From the almanac, the GDOP is derived. On the derived GDOP, the four satellites are so selected that the measuring error is minimized in the selected combination.
From the selected four satellites, the GPS signals are received for collecting ephemerides (data about orbits of the selected satellites). In the case that at least one of the GPS signals is not adequately received from the corresponding satellite due to the adverse effect of an obstacle, the combination of the satellites bringing the GDOP to the second smallest value is selected. Then, the foregoing process is carried out with respect to the selected combination. Next, with a correlation pulse, a pseudo-distance between the GPS receiver and each satellite is derived. To improve the measuring precision, it is better to, use the GPS signals received simultaneously for deriving each pseudo-distance. Hence, the recent prior art GPS receiver is required to provide digital signal processing circuits corresponding to four or more channels.
The data about the pseudo-distances and satellite orbits obtained by the above process are applied to a position deriving circuit for calculating a position of each satellite, solving a quadratic equation, and performing the coordinate conversion, the statistical processing and the like. As a result of performing the processing, the measured position (latitude, longitude and height, for example) of the GPS receiver itself can be obtained.
In such a GPS position measuring system applied for managing a promotion of an automobile, the position data is sent to a writing unit in which the data is written to a memory card (or IC card) together with a measuring time. The position data and the measuring time are sequentially written on the memory card.
The memory card containing the position data and the measuring times accumulated thereon is transferred to the base station. The memory card is inserted to the reading unit for reading the position data and the measuring times from the card. The data is sent to a computer having a operation managing program built therein. In the computer, for example, the one-day operation management of the car is derived from the data.
The foregoing related art provides a circuit for processing a GPS signal and a circuit for deriving the position of the mobile station on each mobile station. It means that each mobile station is required to have a numerically control oscillator (NCO), a digital signal processor (DSP), a 16-bit central processing unit (CPU), a numerical processor and the like. Further, for measuring the position of the mobile station, it is necessary to prepare the four or more channels or use a high-speed IC for operating one channel at high speed. As a result, each mobile station needs large and costly hardware. It is difficult and disadvantageous, in light of the cost, to provide such a mobile station or a GPS receiver in each one of a lot of cars.
Next, the description will be directed to a data format of a GPS signal sent from a satellite. The GPS data is composed of repetitive master frames. One master frame consists of 25 pages (main frames). One page consists of five sub frames. Each sub frame is arranged to have 300 bits. For example, the first sub frame contains an amending coefficient about a timer of the satellite, the second and the third sub frames contain the orbit data of the satellite, and the fourth and the fifth sub frames contain the almanac and an amending coefficient about an ionospheric layer, ant etc.. Since the first to the third sub frames are completed at each page, all the orbit data of the GPS satellite is contained in one page. On the other hand, the fourth and the fifth sub frames are arranged to be completed at each frame master. That is, the remaining part of the almanac, or the orbit data of another GPS satellite is arranged to be dispersed in each page. It means that the orbit data about all the satellites can be obtained over 25 pages.
Since the bit rate is 50 bps, 30 seconds are needed for reading one page. That is, for reading one master frame, 12.5 minutes are needed. It means that for obtaining the almanac, it is necessary to receive a GPS signal for as long a time as 12.5 minutes. Consider a portable GPS receiver. To receive the almanac data, the receiver is required to support its receiving antenna directed toward the sky for 12.5 minutes or more. This is impractical. Further, it takes 12.5 minutes or longer to measure the position of the target object. This is a long time.
To overcome the foregoing shortcomings, it has been considered that the digital signal processing circuits corresponding to the satellites (6 to 8) located within a signal-receivable field are prepared, the one-frame data is received from all the satellites located within the receivable field, and the DGOP is derived by using the data in place of the almanac. To employ this arrangement, it is possible to reduce the receiving time to about 30 seconds. However, it is technically impractical to provide such a number of channels in the receiver in light of the hardware arrangement. If such channels may be provided, the resulting receiver becomes very costly. Hence, this arrangement is impractical, in light of the hardware and the cost.